Steroid hormones are crucial for embryonic development and adult homeostasis in animals. Brassinosteroids (BRs), a special class of plant steroids, have pronounced growth-promoting effects on plants when applied exogenously. Recent genetic studies on dwarf mutants of Arabidopsis, pea, and tomato have confirmed that BRs are important plant hormones that are essential for normal plant growth. Molecular and genetic approaches have been taken to dissect the BR signal transduction pathway, leading to the identification of a few BR-regulated genes and the Arabidopsis BRI1 gene encoding a putative BR receptor. However, the signaling events between BRI1 and various BR-induced cellular responses remains unknown. To understand how BRs are perceived on cell surface and transmitted into cells to regulate many cellular responses, a combination of biochemical, molecular, and genetic approaches will be used to identify additional components of the BR signaling cascade with the following 5 specific aims: (l) Further characterize specific BRI1-interacting proteins identified through a yeast two-hybrid screen and determine the biological and biochemical functions of the most promising BRI1 interactors. (2) Clone the second Arabidopsis BR-insensitive locus, BIN2, and examine its role in BR signaling. (3) Conduct a large-scale genetic interaction screen to identify BR signaling components whose mutations can modify the det2 (a weak BR-deficient mutant) phenotype. (4) Clone at least one of the det2 modifier genes. (5) Perform other genetic screens to identify novel BR signaling mutants. Results of these proposed studies will provide insights into the molecular mechanism(s) by which BRs regulate plant growth and development, thus enhancing our ability to genetically manipulate plant growth to improve crop quality and productivity. in addition, by identifying genes involved in the plant steroid signal transduction, it may be possible to discover previously unidentified signaling components involved in membrane- initiated animal steroid signaling, thereby adding new knowledge to our general understanding of steroid hormone actions and ultimately leading to the identification of more and better targets for treating human diseases.